U.S. patent number 6,784,850 [Application Number 10/096,654] was granted by the patent office on 2004-08-31 for antenna apparatus.
This patent grant is currently assigned to Kabushiki Kaisha Toshiba. Invention is credited to Toshiyuki Haga, Yasushi Iitsuka, Hisao Iwasaki, Tasuku Morooka.
United States Patent |
6,784,850 |
Morooka , et al. |
August 31, 2004 |
Antenna apparatus
Abstract
Power is supplied to antenna elements of a helical antenna
section, which are arranged at an angular distance of 90.degree.
therebetween, with a phase difference of 90.degree.. When a
reception wavelength is .lambda., the height of the helical antenna
section is 0.6 .lambda. to 0.75 .lambda., the number of turns, T,
of each antenna element is about 1, and the pitch angle .alpha. of
each antenna element is 50.degree. to 60.degree.. A straight
portion with a length corresponding to 1/4 of the height of the
antenna section is formed at a location corresponding to 1/2 to 3/4
of the height of the antenna section, the height of the antenna
section is about 0.3 .lambda. to 0.35 .lambda., the number of
turns, T, is about 1, and the pitch angle .alpha. is about
22.degree..
Inventors: |
Morooka; Tasuku (Tokyo,
JP), Iwasaki; Hisao (Tama, JP), Haga;
Toshiyuki (Saitama, JP), Iitsuka; Yasushi (Ageo,
JP) |
Assignee: |
Kabushiki Kaisha Toshiba
(Tokyo, JP)
|
Family
ID: |
19033090 |
Appl.
No.: |
10/096,654 |
Filed: |
March 14, 2002 |
Foreign Application Priority Data
|
|
|
|
|
Jun 27, 2001 [JP] |
|
|
2001-195051 |
|
Current U.S.
Class: |
343/895 |
Current CPC
Class: |
H01Q
11/08 (20130101); H01Q 1/3275 (20130101); H01Q
1/362 (20130101) |
Current International
Class: |
H01Q
11/00 (20060101); H01Q 1/36 (20060101); H01Q
11/08 (20060101); H01Q 001/36 () |
Field of
Search: |
;343/895 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Wimer; Michael C.
Attorney, Agent or Firm: Oblon, Spivak, McClelland, Maier
& Neustadt, P.C.
Claims
What is claimed is:
1. An antenna apparatus comprising: a base plate having one surface
provided with a ground conductor; a helical antenna section
attached to the base plate and provided with an n-number of
antenna-elements helically formed at an angular distance of
(360/n).degree. therebetween, each antenna element including a
straight portion extending parallel to the axis of the helical
antenna section; and a radio frequency signal supplier which is
provided on the other surface of the base plate and supplies power
to the n-number of antenna elements of the helical antenna with a
phase difference of (360/n).degree.; said straight portion having a
lenath corresponding to about one-fourth of the height of the
helical antenna section.
2. An antenna apparatus according to claim 1, wherein said straight
portion is formed at a location corresponding to 1/2 to 3/4 of the
height of the helical antenna section from the ground
conductor.
3. An antenna apparatus according to claim 1, wherein when a
reception wavelength is .lambda., the height of the helical antenna
section is about 0.3 .lambda. to 0.35 .lambda.; wherein the number
of turns of each antenna element is about 1; and wherein each
antenna element of the helical antenna section has a pitch angle of
about 22.degree..
4. An antenna apparatus according to claim 1, wherein when a
reception wavelength is .lambda., the length of each antenna
element is 3/4 .lambda..
5. An antenna apparatus according to claim 1, wherein said helical
antenna section is attached to a surface of the base plate, on
which the radio frequency signal supplier is provided.
6. An antenna apparatus according to claim 1, wherein when a
reception wavelength is .lambda., said ground conductor has a
substantially circular shape with a diameter of 0.5 .lambda. to 1.0
.lambda..
7. An antenna apparatus according to claim 1, further comprising a
low noise amplifier which amplifies a received signal sent from the
helical antenna section via the radio frequency signal
supplier.
8. An antenna apparatus according to claim 1, further comprising a
radome covering at least said helical antenna section.
9. An antenna apparatus according to claim 1, further comprising
fixing means for fixing the antenna apparatus to a movable body.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is based upon and claims the benefit of priority
from the prior Japanese Patent Application No. 2001-195051, filed
Jun. 27, 2001, the entire contents of which are incorporated herein
by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to an antenna apparatus,
and more particularly to an antenna apparatus suitable for an MSB
(Mobile Satellite Broadcast) system.
2. Description of the Related Art
With an increase in the demand for communications, remarkable
developments have been achieved in these years in the field of
communication and broadcasting. In the near future, a mobile
broadcasting system, which makes use of artificial satellites
(hereinafter referred to as "satellites") such as broadcasting
satellites and communications satellites, will be put to practical
use.
In the mobile broadcasting system, it is important that radio waves
from satellites can always be received by mobile stations in good
condition. In big cities, however, high-rise buildings, etc. are
obstacles to radio communication, and mobile stations are unable to
directly receive radio waves from the satellites in most cases. To
solve this problem, there is an idea that relay stations are
disposed at locations open to the satellites without obstacles.
Radio waves from the satellites are re-broadcast to ground areas
via the relay stations. Thereby, the condition of radio reception
by the mobile stations is always kept constant.
In addition, in this system, it is necessary to receive both radio
waves from satellites and radio waves from relay stations with
desired gains. Radio waves come from the satellites at a certain
elevation angle, while radio waves come from the relay stations in
the substantially horizontal direction. These matters, in
particular, have to be considered in designing reception
antennas.
In Japan, for example, a satellite is situated to have an angular
range of 35.degree. to 60.degree. with respect to the zenith
(0.degree.). Thus, a reception antenna is required to have a
performance capable of receiving radio waves from directions at
these angles with a gain of about 2.5 dBi or more. In addition, the
reception antenna is required to have a performance capable of
receiving radio waves coming from the relay stations in a
substantially horizontal direction with a gain of about 0 dBi.
Patch antennas designed for a GPS (Global Positioning System), etc.
may possibly be applied to the antennas for the above system.
However, the conventional antenna apparatus is specifically
designed for receiving radio waves coming from the sky. The
conventional antenna apparatus has a disadvantage that the
reception gain in the horizontal direction is low.
To solve this problem, there is an idea that an additional antenna
for receiving radio waves in the horizontal direction is provided.
According to this solution, however, the cost rises, the size and
weight of the antenna apparatus increase, and the external
appearance is disadvantageously degraded. In particular, these
drawbacks have to be eliminated in the case where the antenna
apparatus is mounted on vehicles, etc.
As has been described above, the conventional antenna apparatus is
specifically designed for receiving radio waves from satellites,
and the gain of horizontal reception in this antenna apparatus is
low. Thus, this antenna apparatus cannot suitably be applied to the
next-generation mobile communication systems. Moreover, since the
conventional antenna apparatus requires a purpose-specific antenna
for obtaining a horizontal reception gain, the size, weight and
cost will disadvantageously increase.
BRIEF SUMMARY OF THE INVENTION
The object of the present invention is to provide an antenna
apparatus at low cost, which is capable of obtaining a reception
gain in a desired range of elevation angles, without increasing the
size and weight of the apparatus.
In order to achieve the above object, this invention may provide an
antenna apparatus comprising: a base plate having one surface
provided with a ground conductor; a helical antenna section
attached to the base plate and provided with an n-number of antenna
elements helically formed at an angular distance of (360/n).degree.
therebetween; and a radio frequency signal supplier which is
provided on the other surface of the base plate and supplies power
to the n-number of antenna elements of the helical antenna with a
phase difference of (360/n).degree..
In this antenna apparatus, it is preferable that the helical
antenna section may comprise a four-wire helical antenna having
four antenna elements arranged at an angular distance of 90.degree.
therebetween, and that the radio frequency signal supplier supply
power to the four antenna elements of the helical antenna section
with a phase difference of 90.degree..
It is preferable that when a reception wavelength is .lambda., the
height of the helical antenna section be 0.6 .lambda. to 0.75
.lambda.. It is also preferable that the number of turns of each
antenna element be about 1. It is also preferable that each antenna
element of the helical antenna section have a pitch angle of about
50.degree. to 60.degree..
In this invention, each of the antenna elements may include a
straight portion extending parallel to the axis of the helical
antenna.
Preferably, the straight portion is formed with a length
corresponding to about 1/4 of the height of the helical antenna
section at a location corresponding to 1/2 to 3/4 of the height of
the helical antenna section from the ground conductor. Preferably,
when a reception wavelength is .lambda., the height of the helical
antenna section is about 0.3 .lambda. to 0.35 .lambda.. More
preferably, the height of the helical antenna section is about 0.34
.lambda.. Preferably, the number of turns of each antenna element
is about 1. Preferably, each antenna element of the helical antenna
section has a pitch angle of about 22.degree..
Moreover, in this invention, the helical antenna section may be
attached to a surface of the base plate, on which the radio
frequency signal supplier is provided.
Additional objects and advantages of the invention will be set
forth in the description which follows, and in part will be obvious
from the description, or may be learned by practice of the
invention. The objects and advantages of the invention may be
realized and obtained by means of the instrumentalities and
combinations particularly pointed out hereinafter.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING
The accompanying drawings, which are incorporated in and constitute
a part of the specification, illustrate embodiments of the
invention, and together with the general description given above
and the detailed description of the embodiments given below, serve
to explain the principles of the invention.
FIG. 1 shows the structure of an MSB system in which an antenna
apparatus according to an embodiment of the present invention is
applied;
FIG. 2 shows the structure of an antenna apparatus according to a
first embodiment of the invention;
FIG. 3 is a view for explaining the effect of reflected waves from
a vehicle 30;
FIG. 4 is a graph showing characteristics required in an antenna
apparatus for use in the MSB system;
FIG. 5 shows characteristics of a four-wire helical antenna shown
in FIG. 2;
FIG. 6 is a graph showing gain characteristics relative to an angle
.theta. when the height H of the antenna is varied, under the
condition that T=1 and D=20 mm;
FIG. 7 is a graph showing the relationship between the diameter D
of the antenna and the directivity;
FIG. 8 is a graph showing the relationship between the number of
turns, T, of the antenna element and the radiation directivity;
FIG. 9 is a graph showing the relationship between the number of
turns, T, of the antenna element and the axial ratio in the range
of angles of .theta.=35.degree. to 60.degree. corresponding to the
coverage range of the satellite;
FIG. 10 is a graph showing the relationship between the pitch angle
.alpha. of the antenna element and the radiation directivity;
FIG. 11 shows the structure of an antenna apparatus according to a
second embodiment of the invention;
FIG. 12 shows the structure of an antenna apparatus according to a
third embodiment of the invention;
FIG. 13 is a graph showing, in comparison, a current distribution
in a conventional helical antenna and current distributions in
monopole antennas relative to their length;
FIG. 14 is a graph showing current distributions of helical
antennas, with the pitch angle .alpha. used as a parameter;
FIG. 15 is a graph showing the relationship between the position of
a straight element and a gain in the case where the height H of the
antenna is H=40 mm;
FIG. 16 is a graph showing the relationship between the position of
a straight element and an axial ratio in the case where the height
H of the antenna is H=40 mm;
FIG. 17 is a graph in which directivity characteristics are plotted
relative to the height H of the antenna shown in FIG. 12;
FIG. 18 illustrates a method of manufacturing a four-wire helical
antenna 6 according to the first and second embodiments;
FIG. 19 illustrates a method of manufacturing a four-wire helical
antenna 6 according to the third embodiment;
FIG. 20 is a graph showing a measured result of an axial ratio
pattern of the helical antenna 6 shown in FIG. 12; and
FIG. 21 is a graph showing a measured result of an input impedance
of the helical antenna 6 shown in FIG. 12.
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of the present invention will now be described with
reference to the accompanying drawings.
FIG. 1 shows the structure of an MSB (Mobile Satellite Broadcast)
system in which an antenna apparatus according to an embodiment of
the present invention is applied. In FIG. 1, radio frequency
signals emitted from a broadcasting satellite 100 are received by a
vehicle 30 and a plurality of gap fillers 21-2n, which are present
on the ground. The gap fillers 21-2n amplify the received signals,
shape the waveforms of the signals, and retransmit the radio
signals to predetermined areas. Thus, a service area of a
predetermined size is created on the ground.
The vehicle 30 can receive radio waves from the broadcasting
satellite 100 at a place open to the broadcasting satellite 100
without obstacles. On the other hand, the vehicle 30 receives radio
waves retransmitted from the gap filler 2n, for instance, at a
shadowy place, i.e. a place not open to the broadcasting satellite
100. The technical requirement in this case is that the gain of
radio wave reception from the broadcasting satellite 100 should be
2.5 dBi or more and the gain of radio wave reception from the gap
filler should be 0 dBi or more. An antenna apparatus 40 according
to the present invention is attached to, for example, to the top
portion of the vehicle 30.
The center frequency of the communication band used in the MSB
system is 2.6425 GHz, which corresponds to a wavelength
.lambda.=115 mm.
First to third embodiments of the present invention will now be
described.
(First Embodiment)
FIG. 2 shows the structure of an antenna apparatus according to a
first embodiment of the invention. The antenna apparatus comprises
a dielectric base plate 1, a helical antenna 6 and a radome 9. The
dielectric base plate 1 is placed on an upper support member 12a.
The helical antenna 6 is attached to the dielectric base plate
1.
Magnets 10 for fixing the antenna apparatus shown in FIG. 2 to the
vehicle 30 are attached to a bottom support member 12b. The number
of magnets 10 is not limited, nor is the size of each magnet 10
limited. It should suffice if the magnets 10 can fix the antenna
apparatus 40 to the vehicle 30 against the wind pressure at the
time of driving.
A ground conductor plate 2 is formed on one surface of the
dielectric base plate 1. The ground conductor plate 2 has a
substantially circular shape. The helical antenna 6 is attached to
that side of the dielectric base plate 1, where the ground
conductor plate 2 is formed. The other side of the dielectric base
plate 2 is provided with a power supply circuit 3, a low noise
amplifier (LNA) 4 and a receiving circuit 5. A shield for
electrically shielding the power supply circuit 3, LNA 4 and
receiving circuit 5 is constituted by the support member 12.
The helical antenna 6 is connected to the power supply circuit 3
via connection pins 7. Radio waves coming to the helical antenna 6
are amplified by the LNA 4 and received by the receiving circuit 5.
The received signal is sent out from the receiving circuit 5 to a
tuner (not shown), etc. over a cable 11.
The helical antenna 6 is a so-called four-wire helical antenna
wherein four antenna elements are arranged on a cylindrical body
with an angular interval (i.e. angular distance) of 90.degree.. The
power supply circuit 6 supplies power to the antenna elements of
helical antenna 6 with a phase difference of 90.degree..
The radome 9 covers the helical antenna 6 and dielectric base plate
1. The radome 9 comprises a cap 9a and a cover 9b. The cover 9b
holds the helical antenna 6 in the fixed state, thus preventing
adverse effect due to vibration, etc. inherent to vehicles. The
shape of the cover 9a is not limited, if the cover 9a can reduce
the adverse effect of wind pressure.
When the dielectric base plate 1 is attached to the support member
12a, the power supply circuit 3, LNA 4 and receiving circuit 5 are
covered by the support member 12a. Preferably, the upper support
member 12aand bottom support member 12b should be subjected to
waterproof treatment to protect electronic parts from rain,
etc.
In the present embodiment, assume that the diameter D of the ground
conductor plate 2 is about 7/10 to 9/10 of a reception wavelength
.lambda.. In the MSB, .lambda.=115 mm, so the diameter d is about
80 to 100 mm. This setting of the size is preferable in order to
minimize the effect due to reflected waves from the roof of the
vehicle 30 when the antenna apparatus 40 is attached to the
roof.
As is shown in FIG. 3, the radio waves coming to the antenna
apparatus 40 include a component coming directly from the
broadcasting satellite 100 (hereinafter referred to as "desired
wave component") and a component reflected by the roof of the
vehicle 30 ("reflected wave component"). The polarization of the
reflected wave component is opposite to that of the desired wave
component. The phase of the reflected wave component is different
from that of the desired wave component. Thus, the reflected wave
component interferes with the desired wave component. This
phenomenon poses a serious problem when the radiation directivity
of the antenna apparatus 40 toward the roof of vehicle 30 is high.
For the antenna apparatus 40 used in the MSB, it is thus necessary
to increase a horizontal radiation level, while minimizing
radiation toward the vehicle.
In general, the radiation directivity of the antenna apparatus
depends greatly upon the size of the ground conductor plate 2
formed on the dielectric base plate 1. If it is supposed that the
size of the ground conductor plate 2 is infinite, the radiation
direction of radio waves is limited to the antenna side, and no
radiation occurs to the ground conductor plate 2 side.
Consequently, the antenna gain in the horizontal direction
decreases. On the other hand, if the size of the ground conductor
plate 2 is decreased, the level of radiation toward the ground
conductor plate 2 increases and the level of radiation to the front
side of the antenna decreases. It is thus understood that the size
of the ground conductor plate 2 has an optimal value.
Experiments conducted by the inventors of the present invention
showed that good characteristics were obtained when the diameter of
the ground conductor plate 2 was about 0.5 to 1.0 .lambda..
The characteristics of the antenna apparatus 40 according to the
embodiment will now be explained referring to FIGS. 4 to 10.
FIG. 4 is a graph showing characteristics required in the antenna
apparatus for use in the MSB system. FIG. 4 demonstrates that a
reception gain of 2.5 dBi or more needs to be obtained in the
angular range of 35.degree. to 60.degree. with respect to the
zenith (0.degree.), and a gain of about 0 dBi needs to be obtained
in the substantially horizontal direction.
FIG. 5 shows the structure and parameters of the four-wire helical
antenna 6. In this embodiment, the parameters shown in FIG. 5 are
also used in the graphs of FIG. 6 and the following. In FIG. 5,
assume that the height of the helical antenna is H, the diameter of
the helical antenna is D, the number of turns of the antenna
element is T, and the pitch angle of the antenna element is
.alpha.. In addition, the diameter of the ground conductor plate 2
is 80 mm. The frequency of received waves is 2.6425 GHz that is the
center frequency. This frequency corresponds to a wavelength
.lambda.=115 mm.
FIG. 6 is a graph showing gain characteristics relative to an angle
.theta. when the height H of the antenna is varied, under the
condition that T=1 and D=20 mm. FIG. 6 demonstrates that the height
H of the antenna needs to be about 70 mm to 85 mm in order to
obtain characteristics required when the angle .theta. is
35.degree., 60.degree.or 90.degree.. This antenna height H
corresponds to 0.6 .lambda. to 0.75 .lambda..
FIG. 7 is a graph showing the relationship between the diameter D
of the antenna and the directivity. FIG. 7 explains that as the
diameter D increases, the radiation in the 180.degree. direction,
i.e. toward the roof of the vehicle 30, increases accordingly. In
short, the larger the antenna diameter D, the greater the effect of
the roof. It is thus preferable to decrease the antenna diameter
D.
FIG. 8 is a graph showing the relationship between the number of
turns, T, of the antenna element and the radiation directivity.
FIG. 8 indicates that the radiation toward the front side increases
as the number of turns, T, increases. Thus, it turns out that the
required value is satisfied if the number of turns, T, is set at
about 1.
FIG. 9 is a graph showing the relationship between the number of
turns, T, of the antenna element and the axial ratio in the range
of angles of .theta.=35.degree. to 60.degree. corresponding to the
coverage range of the satellite. In FIG. 9, too, it is indicated
that a good axial ratio is obtained if the number of turns, T, is
set at about 1.
FIG. 10 is a graph showing the relationship between the pitch angle
.alpha. of the antenna element and the radiation directivity. FIG.
10 demonstrates that as the pitch angle .alpha. is increased, the
gain in the forward direction decreases and the gain in the
90.degree. direction increases. It is also clear that the required
value is satisfied if the pitch angle .alpha. is set at about
50.degree. to 60.degree..
It was thus clarified that in the antenna apparatus 40 of this
embodiment the respective parameters should advantageously be set
at the following values. Specifically, in the antenna apparatus 40
of this embodiment, when the reception wavelength is .lambda., the
height H of the helical antenna section is set at 0.6 .lambda. to
0.75 .lambda.. In addition, the number of turns, T, of the antenna
element is set at about 1, and the pitch angle .alpha. is set at
50.degree. to 60.degree..
By thus setting the parameters, there is provided an antenna
apparatus capable of most efficiently receiving radio waves in the
MSB system in Japan.
(Second Embodiment)
A second embodiment of the present invention will now be described.
FIG. 11 shows the structure of an antenna apparatus 40 according to
the second embodiment of the invention. The antenna apparatus 40
shown in FIG. 11 has the same antenna parameters as the antenna
apparatus 40 shown in FIG. 2. In FIG. 11, the same parts as shown
in FIG. 2 are denoted by like reference numerals.
In FIG. 11, the helical antenna 6 is provided on that side of the
dielectric base plate 1, on which the power supply circuit 3, LNA 4
and receiving circuit 5 are provided. In short, the helical antenna
6 is attached to that side of the dielectric base plate 1, which is
opposite to the side thereof with the ground conductor plate 2.
According to this structure, the power supply circuit 3, LNA 4 and
receiving circuit 5 are covered by the radome 9a, and the upper
support member 12a shown in FIG. 2 can be dispensed with.
Therefore, in this embodiment, compared to the structure of FIG. 2,
the height of the whole antenna apparatus can be reduced.
Specifically, in the structure of FIG. 2, the region including the
power supply circuit 3, LNA 4 and receiving circuit 5 is exposed to
the roof side of vehicle 30, and the height of the whole apparatus
increases by this much. By contrast, according to the present
embodiment, the height of the whole apparatus can be decreased, and
thus the size of the apparatus reduced.
(Third Embodiment)
A third embodiment of the present invention will now be described.
FIG. 12 shows the structure of an antenna apparatus according to
the third embodiment of the invention. In FIG. 12, the same parts
as shown in FIG. 2 are denoted by like reference numerals. The
antenna apparatus 40 shown in FIG. 12 differs from the antenna
apparatus of FIG. 2 with respect to the structure of the helical
antenna 6. In FIG. 12, the helical antenna is denoted by numeral 14
for the purpose of clear distinction.
The helical antenna 14 shown in FIG. 12 includes straight elements
15 as portions of the antenna elements. The straight elements 15
are formed parallel to the axial direction of the helical antenna
14. Specifically, antenna elements of an ordinary helical antenna
are helically wound on a cylindrical body, whereas the straight
elements 15 extend parallel to the axis of the cylindrical
body.
In this embodiment, the length of each straight element 15 is set
at about 1/4 of the height H of the antenna. In addition, the
straight element 15 is situated at a level corresponding to 1/2 to
3/4 of the height of the antenna, relative to the ground conductor
plate 2.
The advantages obtained with the antenna apparatus having the
above-described structure will now be described.
FIG. 13 is a graph showing, in comparison, a current distribution
in a conventional helical antenna (hereinafter referred to as
"conventional model") using the same parameters as mentioned above
and current distributions in monopole antennas relative to their
length. In FIG. 13, the current distribution of the monopole
antenna indicated by #5 is substantially equal to the current
distribution of the conventional model (element length: about 3/4
.lambda.)
It is understood from above that a desired directivity can be
obtained by setting the length of the antenna element of the
helical antenna at about 3/4.lambda.. Table 1 below shows the
relationship between the antenna diameter D and the antenna height
H in a case where the length of the antenna element is set at 3/4
.lambda. and the number of turns of the antenna element is set at
1.
TABLE 1 .alpha. (deg) D (mm) H (mm) 70 9.25 79.87 60 13.53 73.61 50
17.39 65.11 40 20.73 54.64 30 23.43 42.50 20 25.42 29.07
FIG. 14 is a graph showing current distributions of helical
antennas, with the pitch angle .alpha. used as a parameter. In FIG.
14, the current distributions and directivity of the helical
antennas indicated by #2 and #3, whose pitch angles .alpha. are in
the range of 50.degree. to 60.degree. are substantially equal to
the current distribution (#8) of the antenna of the conventional
model.
When the length and the number of turns, T, of the antenna element
are determined, the height of the antenna can be made smaller as
the pitch angle .alpha. decreases. This is advantageous in reducing
the size of the antenna apparatus. However, if the pitch angle
.alpha. is too small, as shown in FIG. 9, desired directivity
cannot be obtained. Thus, in the present embodiment, as shown in
FIG. 12, it is proposed that the straight elements, whose length is
about 1/4 of the antenna height H, should be inserted in the
helical antenna elements.
FIG. 15 is a graph showing the relationship between the position of
the straight element and the gain in the case where the height H of
the antenna is H=40 mm. FIG. 16 is a graph showing the relationship
between the position of the straight element and the axial ratio in
the case where the height H of the antenna is H=40 mm. In both
Figures, the abscissa indicates positions on the straight element.
In addition, in both Figures, symbols #1, #2, . . . ,#7 (ANT#)
indicate positions on the straight element, which are successively
shifted from the bottom side of the antenna in units of 1/8. Table
2 below shows the relationship between ANT# and the position of the
straight element from the bottom side of the antenna.
TABLE 2 ANT# Position 1 0 to 1/4 2 1/8 to 3/8 3 1/4 to 1/2 4 3/8 to
5/8 5 1/2 to 3/4 6 5/8 to 7/8 7 3/4 to 1
FIG. 15 shows that desired gains are obtained at positions #5 and
#6 when .theta.=35.degree., 60.degree. or 90.degree.. As regards
position #6, the gain of backward radiation (i.e.
.theta.=180.degree.) is large. FIG. 16 shows that good axial ratios
are obtained with positions #4 and #5 in the coverage range of the
satellite. It is understood from these results that the required
value is satisfied at the position #5, that is, when the straight
element is inserted at the position corresponding to about 1/2 to
3/4 from the bottom of the antenna.
FIG. 17 is a graph in which directivity characteristics are plotted
relative to the antenna height H in the present embodiment. Table 3
shows the antenna diameter D and pitch angle .alpha. relative to
the antenna height H.
TABLE 3 H (mm) D (mm) .alpha. (deg) 40 29.17 23.58 32 31.05 18.16
24 32.65 13.17 16 34.00 8.52
FIG. 17 demonstrates that a forward gain and a backward radiation
increase when the antenna height H is set at 16 mm. If the antenna
height H is set at 40 mm, a backward radiation level can be
decreased.
In brief, according to the present embodiment, the straight
elements 15 are formed as portions of the antenna elements of the
helical antenna 14. Thereby, the antenna height H can be reduced to
about 40 mm, and further size reduction is achieved. Specifically,
the height H of helical antenna 14 is set at about 0.3 .lambda. to
0.35 .lambda., and the pitch angle .alpha. is set at about
22.degree..
The above-described embodiment, too, can provide an antenna
apparatus capable of efficiently receiving radio waves from the
broadcasting satellite 100 and gap fillers 21 to 2n in the MSB
system. Furthermore, since the height H of the helical antenna can
be set at about 40 mm, a vehicle antenna with a low height and a
good external appearance can be provided, and this vehicle antenna
can withstand an outdoor environment of wind pressure, etc. only by
simple fixation.
Referring to FIG. 18, a method of manufacturing the four-wire
helical antenna 6 used in the first and second embodiments will be
described. In an instance, copper foils each having a suitable
width are formed on one side of a thin flexible base plate 41, as
shown in FIG. 18. The copper foils are arranged with a pitch angle
.alpha. at regular intervals of 1/4 of the circumferential length
calculated by D.times..pi. (D=diameter of antenna 6). The copper
foils function as antenna elements.
A method of manufacturing the four-wire helical antenna 14 of the
third embodiment will be described with reference to FIG. 19. In
FIG. 19, straight elements are formed midway in the copper foils in
the manufacturing process illustrated in FIG. 18. By rolling the
flexible base plate 41 in FIG. 18 (FIG. 19), the four-wire helical
antenna is formed. With these methods, the manufacturing cost of
the four-wire helical antenna can be reduced.
According to these methods, the propagation wavelength in the
direction of the antenna elements of helical antenna 41 can be made
shorter than the reception wavelength .lambda. in accordance with
the kind of dielectric material of the flexible base plate 41.
Experimental data confirmed that when copper foils with a thickness
of 35 .mu.m were formed on a flexible base plate made of PET
(polyethylene terephthalate) with a thickness of 100 .lambda.m, the
propagation wavelength was about 90% of the reception wavelength
.lambda.. In short, the size of the antenna section can be further
reduced by properly selecting the dielectric material of the
flexible base plate 41.
It was also confirmed by an experimental result that good axial
ratio characteristics were obtained by setting the number of turns,
T, at about 1. FIG. 20 is a graph showing a measured result of the
axial ratio pattern of the helical antenna 6. It is understood from
FIG. 20 that desired directivity and axial ratio were obtained.
In the embodiments described above, power is supplied to the
antenna elements of the helical antenna, which are arranged with an
angular distance of 90 therebetween, with a phase difference of
90.degree.. When the reception wavelength is .lambda., the height H
of the helical antenna section is set at 0.6 .lambda. to 0.75
.lambda., the number of turns, T, of each antenna element is set at
about 1, and the pitch angle .alpha. is set at 50 to 60.degree.. In
addition, the diameter of the ground conductor plate 2 provided on
one side of the dielectric base plate 1 on which the helical
antenna is attached is set at 0.5 .lambda. to 1.0 .lambda..
Moreover, the helical antenna is attached to the dielectric base
plate 1 on the same side as, or on the opposite side to, the ground
conductor plate 2.
In the third embodiment, the straight elements each with a length
corresponding to about 1/4 of the height H of the antenna are
incorporated in the respective antenna elements of the four-wire
helical antenna at locations corresponding to 1/2 to 3/4 from the
ground conductor plate. In this case, the height H of the helical
antenna section is set at about 0.3 .lambda. to 0.35 .lambda., the
pitch angle .alpha. is set at about 22, and the number of turns, T,
of each antenna element is set at about 1.
With the above structure, both radio waves from the satellite and
radio waves coming in the substantially horizontal direction can be
received without increasing the area or the size of the antenna
apparatus. Furthermore, since the level of radiation directivity
toward the vehicle is suppressed, a good reception sensitivity can
be obtained.
FIG. 21 is a graph showing a measured result of an input impedance
of the helical antenna 6 according to this embodiment. As is shown
in FIG. 21, with a center frequency of 2642.5 MHz, an impedance of
41.7+j0.7 .OMEGA. was obtained. This value is close to 50 .OMEGA.,
which is an ordinary value for a power supply line of an antenna
apparatus.
In this embodiment, the length of the antenna element of the
helical antenna is set at about 3/4 .lambda.. With this length of
the antenna element, the impedance of about 50 .OMEGA., which is an
ordinary value for a power supply line of an antenna apparatus, is
obtained. Thereby, good matching with the power line is attained
without the need to use a special matching circuit.
According to the structures of the second and third embodiments,
the height of the antenna can be reduced, and the volume (size) of
the antenna decreased. Thus, the reduction in size and cost can be
achieved. Furthermore, there is provided an antenna apparatus which
has a good external appearance and can withstand an outdoor
environment of wind pressure, etc.
Additional advantages and modifications will readily occur to those
skilled in the art. Therefore, the invention in its broader aspects
is not limited to the specific details and representative
embodiments shown and described herein. Accordingly, various
modifications may be made without departing from the spirit or
scope of the general inventive concept as defined by the appended
claims and their equivalents.
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